The glass-ceramics which contain a solid solution of β-quartz or of β-spodumene (solid solutions of β-quartz and β-spodumene), as main crystalline phase(s) are materials which are known per se, and which are obtained by heat treatment of glasses or inorganic fillers. These materials are used in various contexts and notably as a substrate for cook-tops and as fire-windows.
Transparent, opalescent, or even opaque glass-ceramics are known of various colors.
The preparation of articles made from a glass-ceramic of β-quartz and/or β-spodumene classically comprise three main successive steps:                a first step of melting an inorganic glass or a filler, which is a precursor of such a glass, which is generally carried out between 1,550 and 1,650° C.,        a second step of cooling and shaping the molten glass obtained, and        a third step of crystallization or ceramming of the cooled, shaped glass, by an appropriate heat treatment.        
Upon completion of the first step of melting, it is important to remove gaseous inclusions as efficiently as possible from the molten glass mass. To this end, at least one fining agent is incorporated within it.
Arsenic oxide (As2O3) is generally used in the methods used hitherto, typically at more than 0.1% by weight and at less than 1% by weight. Antimony oxide (Sb2O3) is also used at higher contents.
In view of the toxicity of these products and of the most drastic rules in force (with reference to the safety and the protection of the environment), the incorporation of these products is sought to be minimized, even avoided, and other compounds are sought which are less toxic, even non-toxic and which are effective as fining agents.
Furthermore, for obvious reasons of economy, it is not desired to modify the operating conditions of the industrial method made use of at present. Notably, it is not desired to operate at higher temperature, which would imply spending more energy and would worsen the problems of corrosion.
Compounds other than arsenic oxide and antimony oxide are thus sought after which are effective under the same operating conditions as fining agents (compounds substituting, at least partially, advantageously totally, for said oxides) of the glass which is to be cerammed.
In addition to its role as fining agent, the arsenic oxide can be incorporated to confer a dark color to the glass-ceramic which contains it. To this end, it reacts with the vanadium present; it reduces said vanadium during the ceramming. In the precursor glass, the vanadium which is present—it is in general added at the rate of about 0.2% by weight—is so mainly in the oxidized state. However, during the ceramming, the reaction between the arsenic and the vanadium is never complete. The reaction has a tendency to continue when the glass-ceramic is heated further. Thus, a decrease is observed of the transmission in the visible and the infra-red when the glass-ceramic undergoes a treatment, known as <<aging>>, of 100 hours at 700° C. In view of the foregoing, it is strongly desired that the compounds substituting for arsenic oxide, which are proposed as agents for fining the glass, do not disrupt obtaining, after ceramming, the dark color, when it is desired, and it would be moreover advantageous that they ensure a better stability of said dark color with ageing.
According to prior art, it has been proposed to use, as agent for fining glasses, precursors of glass-ceramics (containing a solid solution of β-quartz or of β-spodumene (solid solutions of β-quartz and β-spodumene), as main crystalline phase(s)), tin oxide (SnO2).
Patent Applications JP 11 100 229 and 11 100 230 thus describe the use of tin oxide (SnO2), alone or in combination with chlorine (Cl), at the rate of:                SnO2: 0.1-2% by weight        Cl: 0-1% by weight.        
Applications DE 19 939 787.2 and WO 02/16279 mention the use of tin oxide (SnO2), cerium oxide (CeO2), and sulfate or chlorine-containing compounds. These documents more particularly illustrate the use of tin oxide which is incorporated at less than 1% by weight. No specification on the fining performance obtained is found in said documents.
The inventor, faced with this technical problem of providing fining agents which substitute, at least partially, for As2O3 and/or Sb2O3, has studied the performances of SnO2 and has shown that this compound is not fully satisfactory alone.
The effectiveness of SnO2, as agent fining glasses precursors of glass-ceramics, increases with the amount of said SnO2 incorporated. It is thus possible to obtain good results with regard to the fining of said glasses, which good results are almost comparable to those obtained hitherto notably with As2O3, by using adequate amounts of SnO2. The incorporation of these adequate amounts, which are effective from a fining point of view, is, however, detrimental:                firstly, due to the low solubility of SnO2 in the glass. Problems of devitrification and difficulties of implementation of the melting are observed very quickly, and        secondly, due to the reducing power of SnO2. SnO2 can reduce transition metal oxides which are present in the glass, notably vanadium oxide which is commonly used to produce dark glass-ceramics and therefore to strongly influence the color of the ceramic sought after. In its presence, in the amounts which are effective for the fining of the precursor glass, the color of the final glass-ceramic is difficult to control.        
It is thus hardly sufficient to propose using SnO2 as effective fining agent instead of the conventional fining agents (As2O3 and/or Sb2O3).
The inventor, with reference to the technical problem, has tested the joint use of SnO2 and Cl. The results obtained are much less interesting than those obtained with the means of the invention which are described below (vide Table 1, infra).
According to prior art, the joint incorporation has been described of fluorine and an oxide of a multivalent element (an element which can exist in various valencies) in compositions of glass or glass-ceramics. Such an incorporation has not, to the knowledge of the inventor, been described with reference to the problem of fining glasses, which are precursors of glass-ceramics of β-quartz and/or β-spodumene.
Thus, U.S. Pat. No. 6,673,729 describes glass-ceramics of β-quartz and/or of keatite, which can contain fluorine (from 0 to 0.6% by weight) and at least one agent for fining the glass (generally from 0.5 to 2% by weight), which is notably selected from As2O3, Sb2O3, SnO2, CeO2 . . . Only the use of As2O3 and Sb2O3 is illustrated in the Examples. The compositions indicated for these glass-ceramics, which are intended to constitute substrates, which are used for example for preparing reflectors, are strict (note may be made in particular of: Na2O+K2O: 0.5 to 3% by weight, MgO+ZnO<0.3% by weight and Fe2O3<0.02% by weight). Said glass-ceramics have a strong transmission in the near infra-red and, at the surface, a layer which is glassy and has a certain roughness. The presence of fluorine reveals to be advantageous for decreasing the viscosity, at high temperature, of the precursor glasses and eventually for increasing the mechanical strength of the glass-ceramics.
It is to the merit of the inventor to have demonstrated, in such a context, the interest of a “fluorine+at least one oxide of a multivalent element (at least one oxide of an element which can exist in various valences)” combination, to have observed surprisingly that such a “fluorine+at least one oxide of a multivalent element” combination is effective as a fining agent, the SnO2 being incorporated, when it does, at a low content in said combination, such that the problems set forth above are minimized, even avoided. It is also to the merit of the inventor to have observed that such a combination, which is effective as a fining agent, is also effective with reference to the technical problem of the stability of the color.